1. Field of the Invention
The present invention relates to a light emitting diode backlight unit, and more particularly, to a light emitting diode backlight unit that supplies light to a liquid crystal display panel through a plurality of light emitting diodes (LEDs) and a liquid crystal display (LCD) device using the same.
2. Discussion of the Related Art
In general, an LCD device is a display device displaying a desired image by individually providing data signals corresponding to image information to pixels arranged in a matrix form and controlling the light transmittance of the pixels.
Accordingly, the LCD device includes both an LCD panel where pixels are arranged in the matrix form and a driving unit for driving the pixels.
The LCD panel includes a thin film transistor array substrate and a color filter substrate which face each other and are attached such that they have a uniform cell-gap therebetween and a liquid crystal layer in the cell-gap.
A common electrode and a pixel electrode are formed on the LCD panel in which the array substrate and the color filter substrate are attached to each other in order to apply an electric field to the liquid crystal layer.
When a voltage of a data signal supplied to the pixel electrode is controlled such that a voltage is supplied to the common electrode, liquid crystal in the liquid crystal layer rotates due to a dielectric anisotropy according to the electric field formed between the common electrode and the pixel electrode. As a result, light is transmitted or blocked in each pixel thereby displaying a character or an image.
The LCD device cannot emit light on its own and instead displays an image by controlling the transmittance of light. As such, a supplementary unit such as a backlight unit for irradiating light onto the LCD panel is required.
There are two types of backlight units, a side-type and a direct type. In a side-type backlight unit, lamps are disposed at one side surface or at both side surfaces and light is reflected, diffused, and collected through a light guide plate, a reflector, and optical sheets and is then transmitted to a front surface of the LCD panel. In a direct-type backlight unit, lamps are disposed at the rear of an LCD panel such that light is directly transmitted to a front surface of the LCD panel.
FIG. 1 is a perspective view illustrating a side-type backlight unit. The side-type backlight unit includes a light guide plate 5 formed of a transparent material and disposed at the rear of an LCD panel (not illustrated), a lamp 20 disposed at the side of the light guide plate 5, a reflector 30 disposed at the rear of the light guide plate 5; a lamp reflector 25 for fixing the lamp 20 to the side of the light guide plate 5 and reflecting the light of the lamp 20 to the side of the light guide plate 5 and a line 26 for supplying power to the lamp 20.
Light generated from the lamp 20 is incident on the side of the light guide plate 5. The reflector 30 disposed at the rear of the light guide plate 5 reflects light being transmitted to the rear surface of the light guide plate 5 toward an upper surface of the light guide plate 5 thereby reducing light loss and improving uniformity.
Thus, the light guide plate 5 together with the reflector 30 transmits the light generated from the lamp 20 toward the upper surface.
FIG. 2 is a perspective view schematically illustrating a direct-type backlight unit. The direct-type backlight unit includes a reflector 30 disposed at the rear surface of an LCD panel (not illustrated), a plurality of lamps 20 disposed at an upper portion of the reflector 30 which makes light incident upon the overall rear surface of the LCD panel, a diffusion plate 40 covering the lamps 20 and diffusing light generated form the lamps 20 and lines 26 for supplying power to the lamps 20.
In general, a cold cathode fluorescent lamp (CCFL) has a tubular shape which has a length corresponding to a distance between sides of the LCD panel. A CCFL is used as the light source in a side-type backlight unit or the direct-type backlight unit. The CCFL generates white light when power is supplied through the wire 26 at both ends.
However, mercury used in the CCFL as a fluorescent material is harmful to the human body, thus, the CCFL may not satisfy environmental regulations as these regulations gradually tighten.
In addition, strides in the development of an LCD device using a time division method in which one frame of an image is time-divided into a plurality of sub-frames and red, green and blue lights are sequentially supplied to the plurality of sub-frames in order to improve the transmittance and color rendering capability of an LCD device have been recently made. However, using CCFL in a LCD device limits the application of the time division method.
Recently, a light emitting diode (LED) has been used as a backlight of the LCD device. A LED has a longer life span than the CCFL and operates on 5V. Therefore, the LED does not require an additional inverter.
A high-brightness LED has a longer life span than the related art CCFL and consumes only about 20% of the electric power required by previously available applications. Since additional equipment, such as an inverter, is not required, the high-brightness LED has advantages in that products using this type of LED have a low profile and have a more efficient inner area. In addition, the high-brightness LED has superior color generating capabilities to that of a CCFL. Furthermore, mercury controls will begin to take effect, thus increasing the demand for adopting a light emitting diode backlight.
A light emitting diode, a next generation light source which is more energy efficient than previously available light sources and can be used almost semipermanently, has been used as a main light source for mobile devices such as cellular phones, digital cameras, personal digital assistants (PDAs) and the like.
FIG. 3 is a view illustrating a typical light emitting diode backlight unit and a structure of a direct-type light emitting diode backlight using a light emitting diode.
As illustrated therein, a plurality of light emitting diodes 20A to 20C generating colors of red (R), green (G), and blue (B) are mounted onto a light emitting unit printed circuit board (PCB) 50. In addition, a driving unit printed circuit board 60 onto which drive components (not illustrated) for driving the LEDs are mounted is constructed in the form of another PCB outside the light emitting diode backlight.
Furthermore, a control unit printed circuit board 70 which outputs signals that control the drive components is electrically connected to the driving unit PCB 60.
A reference numeral 10 denotes a light emitting diode backlight in which the light emitting unit PCB 50 is located.
In the typical light emitting diode backlight structure having such a construction, a long signal line for transmitting a signal between the light emitting diode and the driving unit PCB is required. A long signal line causes a serious voltage drop in the LED backlight circuit. The brightness of the LED changes according to electric current. As such, the voltage drop minimizes the efficiency of the LED backlight. In addition, since additional processes are required such as designing the additional driving unit PCB and coupling with the light emitting printed circuit board, productivity is decreased.